Photosystem II (PS II), D2 subunit. PS II is a multi-subunit protein found in the photosynthetic membranes of plants, algae, and cyanobacteria. It utilizes light-induced electron transfer and water-splitting reactions to produce protons, electrons, and molecular oxygen. The protons generated are instrumental in ATP formation. Molecular dioxygen is released as a by-product. PS II can be described as containing two parts: the photochemical part and the catalytic part. The photochemical portion promotes the fast, efficient light-induced charge separation and stabilization that occur when light is absorbed by chlorophyll. The catalytic portion, where water is oxidized, involves a cluster of Mn ions close to a redox-active tyrosine residue. The Mn cluster and its ligands form a functional unit called the oxygen-evolving complex (OEC) or the water-oxidizing complex (WOC). The D1 and D2 subunits are a pair of interwined polypeptides. They contain all the cofactors involved directly in water oxidation and plastoquinone reduction. The D1 subunit contains the Mn cluster that constitutes the site of water oxidation. D1 and D2 are highly homologous and are also similar to the L and M proteins in bacterial photosynthetic reaction centers.
Jiyao W et al.(2023). "The conserved domain database in 2023.",